DESCRIPTION (Applicant's abstract): This proposal will examine the role that regulatory G protein signaling (RGS) proteins play in modulating G-protein linked signal transduction in vivo. RGS proteins are a novel family of proteins that increase the hydrolysis of GTP bound to the G-alpha subunit and reduce the activation of the second messenger system. Thus, changes in the levels of RGS proteins may mediate increases or decreases in the responsiveness of G protein-coupled receptors. We have demonstrated that overexpression of one particular RGS protein, RGS4, can induce desensitization of one member of the 5-HT2 receptor family in cells in culture. However, it is not known whether RGS4 protein has the same effect on 5-HT2 receptors in mammalian brain in vivo. Antisense oligodeoxynucleotide strategies will be used to test the hypothesis that RGS4 protein regulates 5-HT2A receptor signaling in the hypothalamus. We will test the regulation of 5-HT2A receptor adaptation in the hypothalamic paraventricular nucleus, because it contains abundant levels of RGS4 protein and 5-HT2A receptors, 5-HT2A receptors in this nucleus can be desensitized or become supersensitive with several treatments, and because hormone responses to agonist treatment can be used as a sensitive measure of receptor signaling in the hypothalamus. We will use antisense oligonucleotide strategies to reduce the levels of RGS4 protein to 1) cause supersensitivity of signaling and 2) attenuate the desensitization of 5-HT2A signal transduction, induced by chronic treatment with the 5-HT2A/2C agonist DOI. Lastly, our preliminary data suggest that RGS4 protein expression is decreased during fluoxetine-induced 5-HT2A receptor supersensitivity. Our studies have ruled out several possible mechanisms and have led to the hypothesis that a decrease in the levels of RGS4 proteins could mediate this supersensitivity. Thus, we will examine the correspondence between the time course of the reduction in RGS4 protein expression and of fluoxetine-induced 5-HT2A receptor supersensitivity. The results of the proposed studies will provide new insight into the function of RGS proteins in vivo, and the mechanisms regulating 5-HT2A receptor systems. Given the importance of G protein signaling in mammalian brain, it is essential to understand the regulation of RGS proteins. Because of the prominent role that 5-HT2A receptors play in the etiology of neuropsychiatric disorders, understanding the regulation of adaptive changes in 5-HT2A signaling by RGS proteins will lay the foundation for new targets for therapeutic intervention for these disorders.